One interesting result is that the product compositions do not return exactly to their design values for these feed flowrate changes, despite the fact that the reflux ratio is held constant. In theory, if the flowrates are all held as ratios and the temperature on any tray is held constant, all temperatures and compositions throughout the column should return to their original values.
However, the pressures throughout the column are not constant in an Aspen Dynamics simulation. Therefore, holding stage 23 at a constant temperature does not keep all compositions constant because the pressure on stage 23 is not constant. For example, the initial pressure is 126,621 Pa. For the 20% increase in feed flowrate, the higher liquid and vapor rates in the column give a higher final pressure on stage 23 (128,686 Pa). For the 20% decrease in feed flowrate, the lower liquid and vapor rates in the column give a lower final pressure on stage 23 (124,807 Pa). The water impurity in the distillate seems to be affected the most.
Figure 8.3 gives results for three feed composition disturbances. In the first (solid lines), the feed composition is changed from 30/40/20 to 35/45/20 mol% MeAc/MeOH/H2O. In the second (dashed lines), the feed composition is changed to 25/55/20 mol%. In the third (dotted lines), the feed composition is changed to 30/40/30 mol%.
The increase in the water content of the feed has the most effect on product purities. The distillate water impurity increases to almost 1 mol%.
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